1. Field of the Invention
The present invention relates to a method of forming a microstructure and, more specifically, to an improvement in a method of forming microstructures such as the LIGA method, an improvement in the method of forming microstructures applying patterning techniques commonly used for semiconductor processing and the like, and to an X-ray mask.
2. Description of the Background Art
Recently, microprocessing techniques for forming highly miniaturized structures by using methods known for manufacturing semiconductor integrated circuit devices have been studied intensively. Especially the LIGA (Lithograph Galvanformung und Abformung) method for forming microstructures having a high aspect ratio is attracting much attention (see Nikkei Mechanical 1990. Nov. 26, pp. 72-79 and Kikai Sekkei Vol. 3, No. 6 (May, 1991), pp. 26-28). The LIGA method can be applied to manufacturing micro machines, optical elements, sensors, actuators and the like, and the applicable field thereof is very wide.
In the conventional LIGA method, polymethylmethacrylate (PPMA) is mainly used as a resist in the step of forming a resist pattern by using X-ray lithography.
FIGS. 1 to 8 are cross-sections schematically showing basic steps of the conventional LIGA method. First, in the step shown in FIG. 1, typically, a resist layer 102 of polymethylmethacrylate (PMMA) is formed to a desired thickness (about several 10 .mu.m to several 100 .mu.m) on a substrate 101. Then, an X-ray mask 100 having a desired pattern 100a is used to expose resist layer 102 with synchrotron orbital radiation (SOR). Then, in the step shown in FIG. 2, resist layer 102 which has been exposed in the step of FIG. 1 is developed to form a resist pattern 103. In the step shown in FIG. 3, substrate 101 having resist pattern 103 formed thereon is dipped in a plating liquid, and Ni, Cu, Au or the like is deposited on depressed portions of resist pattern 103 on substrate 101 by electroplating, thus forming a metal structure 104. In the step shown in FIG. 4, the substrate 101 and the resist are removed to obtain metal structure 104. Metal structure 104 is used as a mold, and dielectric plastic is filled into the metal structure 104 by injection molding, and thus an opposite mold 105 of dielectric plastic is formed. Then, in the step shown in FIG. 5, in accordance with the mold 105 of dielectric plastic fabricated in the step of FIG. 4, a plastic mold 108 of dielectric plastic 106 and a conductive plastic sheet 107 is formed. Then, in the step shown in FIG. 6, mold 105 is removed to separate or release the plastic mold 108. Then, in the step shown in FIG. 7, a metal 109 is deposited on the depressed portion of plastic mold 108 by electroforming. In the step shown in FIG. 8, the plastic 108 is removed, and thus a miniaturized metal structure 110 is obtained. The term "microstructure" used in this specification includes, but is not limited to, metal structure 104 formed in the step of FIG. 3, mold 105 formed in the step of FIG. 4, plastic mold 108 formed in the step of FIG. 5, and metal structure 110 formed in the step of FIG. 8.
The degree of integration of semiconductor integrated circuits has been greatly increased these days, and as the degree of integration becomes higher in VLSIs (Very Large Scale Integrated Circuits), it becomes necessary in the field of manufacturing semiconductor integrated circuits and the like to miniaturize the element structures of the semiconductor devices. To meet this demand, an X-ray (having a wavelength of about 10 .ANG.) lithography method using light having a shorter wavelength than the conventional photolithography employing ultraviolet rays (having a wavelength in the range of about 3000 .ANG. to about 5000 .ANG.) has attracted attention.
FIG. 9 is a cross-section schematically showing an X-ray mask used in the known X-ray lithography method in patterning such a conventional semiconductor integrated circuit. Referring to FIG. 9, the X-ray mask 200 includes a silicon substrate 201 having an opening 201h at a central portion thereof, an X-ray transmitting membrane 202 provided to cover one surface 201a of silicon substrate 201, and an X-ray absorber film 203 having a desired pattern formed on the surface of X-ray transmitting membrane 202. X-ray transmitting membrane 202 is formed of a material which transmits X-rays, that is, a material having a high transmittance of X-rays, such as silicon nitride (SiN) or silicon carbide (SiC).
Meanwhile, X-ray absorber film 203 is formed of a material which absorbs most X-rays, that is, a material having a low X-ray transmittance such as tungsten (W) or tantalum (Ta). X-ray absorber film 203 is supported by X-ray transmitting membrane 202. On the other surface 201b of silicon substrate 201, a film 204 of silicon nitride (SiN), silicon carbide (SiC) or the like is formed. The average thickness of X-ray absorber film 203 in the conventional X-ray mask is about 0.7 .mu.m.
The method of manufacturing a conventional photomask will be described in the following. FIGS. 10 to 17 are cross-sections schematically showing the steps of manufacturing a conventional X-ray mask. First, in the step shown in FIG. 10, X-ray transmitting membrane 202 of silicon nitride (SiN), silicon carbide (SiC) or the like having a thickness of about 2 .mu.m is formed on one surface 201a of silicon substrate 201 by chemical vapor deposition (CVD) or physical vapor deposition (PVD). In this step, a film 204 of silicon nitride (SiN), silicon carbide (SiC) or the like to serve as an etching mask for etching back silicon substrate 201 in the subsequent step (described below) is formed on a prescribed region on the other surface 201b of silicon substrate 201 by chemical vapor deposition or physical vapor deposition. In the step shown in FIG. 11, an X-ray absorber film 203a of tungsten (W), tantalum (Ta) or the like is formed by chemical vapor deposition or physical vapor deposition on the surface of X-ray transmitting membrane 202. The thickness of X-ray absorber film 203a is about 0.7 .mu.m. In the step shown in FIG. 12, a resist layer 205 is formed by spin coating a resist material on the surface of X-ray absorber film 203a.
A method of applying a resist on the surface of X-ray absorber film 203a by spin coating will be described by way of example. First, a prescribed amount of the resist material is dropped from a nozzle onto the surface of X-ray absorber film 203a and left as it is for a while, so that the resist spreads on the surface of X-ray absorber film 203a. Then, by rotating silicon substrate 201 at a high speed by using a spinner or the like, excessive resist material is scattered and spread, thereby forming a resist layer 205 having uniform thickness. The thickness of resist layer 205 formed on the surface of X-ray absorber film 203a is normally 3 .mu.m or less, though it depends on the viscosity of the applied resist material, the speed and number of rotations of the spinner, and the type and speed of gasification of the gasifier of the solvent.
Then, in the step shown in FIG. 13, a pattern is drawn on resist layer 205 (i.e., the resist layer is exposed) to provide a desired mask absorber by using electron beam 206 emitted from an electron gun (an electron beam emitting apparatus, which is not shown). In the step of FIG. 13, when a resist for photolithography is used, resist layer 205 may be exposed with an ultraviolet ray (having a wavelength in the range of about 3000 .ANG. to about 5000 .ANG.) to provide a desired mask absorber.
In the step shown in FIG. 14, by developing the exposed resist layer 205 and removing portions 205b, a desired resist pattern 205a is formed. In the step shown in FIG. 15, using resist pattern 205a as a mask, areas of X-ray absorber film 203a that are exposed through opening portions 205b are etched so that X-ray absorber film 203a is penetrated, by dry etching such as reactive ion etching (RIE). Then in the step shown in FIG. 16, the resist layer 205a is removed, the film 204 is used as an etching mask, and silicon substrate 201 is removed by etching (back etching) from the other surface 201b of the substrate 201 up to X-ray transmitting membrane 202 by dry etching such as reactive ion etching, wet etching or the like, and thus an X-ray mask 200 is formed (see FIG. 17).
Again referring to FIG. 13, by using electron beam 206 or an ultraviolet ray, resist layer 205 can be exposed with sufficiently high precision in a desired shape to the depth of at most 2 .mu.m in the thickness or depth direction of resist layer 205. The reasons for this are that the electron beam or the ultraviolet ray is absorbed by the resist and the precision of the resist pattern is decreased by diffraction of light, and so on.
Referring again to FIG. 15, in the step of etching X-ray absorber film 203a by using resist pattern 205a as a mask, in order to etch X-ray absorber film 203a deeply in the thickness or depth direction of the membrane 203a, it is necessary to increase the selection ratio represented by the following equation (1): ##EQU1## However, though various etching reaction gases are used, there is a limit to increasing the selection ratio.
As mentioned above, the thickness of the X-ray absorber film of the conventional X-ray mask is about 0.7 .mu.m. The reason why this is so will now be described. First, in the conventional, method of manufacturing an X-ray mask shown in FIGS. 10 to 17, it was difficult to make the X-ray absorber film thicker than 0.7 .mu.m. More specifically, in the conventional method of manufacturing the X-ray mask, the resist layer is formed by spin coating, and therefore it is difficult to make the resist layer thick. In addition, it was difficult to form a deep resist pattern with a high precision in the depth direction of the resist layer due to diffraction of light, insufficient intensity of the exposure light source and so on, because the resist layer is exposed by an electron beam or an ultra violet ray. In addition, there is a limit to increasing the selection ratio of the etch rate of the resist layer relative to the etch rate of the X-ray absorber film, as described above.
As a second reason, in the field of manufacturing semiconductor devices, light having a wavelength in the range of about 10 .ANG. to about 4000 .ANG. is generally used in the step of exposure during lithography. Therefore, when the X-ray absorber film is thick, the resolution of patterns to be transferred onto the resist layer is decreased by the diffraction effect of the light.
In accordance with the conventional method of manufacturing the X-ray mask shown in FIGS. 10 to 17, even if the selection ratio is increased, the thickness of the X-ray absorber film which can be patterned is at most 3 .mu.m.
In lithography used in the process of manufacturing semiconductors, it is known to use a copolymer resist including methylmethacrylate (MMA) and methacrylic acid (MAA) (see J. Electrochem., Soc., Vol. 1126, No. 1 (1979), pp. 154.about.161).
In the step of forming a resist layer on the semiconductor substrate during lithography used in the process of manufacturing semiconductors, normally spin coating is employed.
A method of applying a copolymer resist including methylmethacrylate and methacrylic acid units on a semiconductor substrate will now be described by way of example.
In lithography employed in the process of manufacturing semiconductors, the copolymer resist including methylmethacrylate and methacrylic acid units is produced by polymerizing methylmethacrylate monomer and methacrylic acid monomer in a solvent such as toluene. The copolymer including methylmethacrylate and methacrylic acid units produced in the solvent is then precipitated by methanol or the like. The copolymer is next purified, and then dissolved in a solvent such as ethyl cellosolve acetate (ECA). The solution including the copolymer is dropped onto a semiconductor substrate and applied to have a prescribed uniform thickness on the substrate by spin coating. Then, the substrate on which the solution has been applied is subjected to thermal processing to remove the solvent. Thus, a copolymer resist layer including methylmethacrylate and methacrylic acid units is formed on the semiconductor substrate. The thickness of the resist layer formed in this manner on the semiconductor substrate is 3 .mu.m or less.
Now, in the field of forming microstructures using the LIGA method and the like described above, it is inevitable that long periods of exposure using a large-scale high-performance synchrotron orbital radiation apparatus having high light intensity would be used, because the PMMA resist does not absorb X-rays very much. In order to obtain a resist pattern having a high resolution, the diffraction effect of the light must be decreased, and in order to realize a deeper exposure in the vertical direction of about several 10 .mu.m or more, synchrotron orbital radiation of short wavelength about 2 .ANG. to about 5 .ANG. is necessary.
FIG. 18 is a graph showing a relation between the thickness or depth of the resist that can be patterned for a resist layer including generally employed polymethylmethacrylate (PMMA) and the wavelength of the X-ray. Referring to FIG. 18, theoretically, it is possible to pattern deeply with high precision in the thickness or depth direction of the resist layer by thickly forming the resist layer, using light of short wavelength and reducing the diffraction effect of light.
Therefore, especially in the field of the LIGA method, in order to form microstructures having a high aspect ratio, a synchrotron orbital radiation apparatus which can emit synchrotron orbital radiation of short wavelength of about 2 .ANG. to about 5 .ANG. with a high light source intensity for a long period of time has been desired. However, use of such synchrotron orbital radiation apparatus is generally difficult.
Microstructures such as micro machines, optical elements, sensors, actuators and the like must have sufficient mechanical strength, and for this reason, it is necessary to form structures having portions that are finely processed on the order of .mu.m dimensions and having a high aspect ratio. Such microstructures have various thicknesses dependent on the applications and purposes of such structures, and normally the thickness varies in the range of about 1 .mu.m to 1 mm. When a microstructure such as a micro machine is to be formed by the LIGA method, it is necessary to make the resist layer 102 thick, e.g. several 10 .mu.m to several 1,000 .mu.m, to form deep resist patterns in the thickness or depth direction of the resist layer, as compared with the manufacturing of the semiconductor integrated circuits, as can be seen from FIG. 1. Therefore, when microstructures are to be formed by the LIGA method or the like, it is necessary to carry out long duration exposures with synchrotron orbital radiation having a short wavelength of about 2 .ANG. to 15 .ANG. by using the X-ray mask 100, to reduce the diffraction effect of the light and to expose the resist layer deeply in the thickness or depth direction.
FIG. 19 is a graph showing a relation between the film thickness of the necessary X-ray absorber used for the X-ray mask in manufacturing micro machines and the like by the LIGA method and the wavelength of the X-ray. Referring to FIG. 19, when micro machines and the like are to be manufactured by the LIGA method, the average thickness of the X-ray absorber of the X-ray mask must be at least 5 .mu.m. More specifically, the conventional X-ray mask used in manufacturing semiconductor integrated circuits has an X-ray absorber with average thickness of about 0.7 .mu.m so that when the resist layer 102 is irradiated with synchrotron orbital radiation for a long period of time using the X-ray mask 100, the synchrotron orbital radiation passes through the X-ray absorber film 100a, causing exposure of portions of the resist layer 102 that are not intended to be exposed. Therefore, the desired resist pattern cannot be obtained.
For manufacturing micro machines and the like by the LIGA method, an X-ray mask having an X-ray absorption membrane which can sufficiently intercept the synchrotron orbital radiation even when it is exposed to the synchrotron orbital radiation for a long time, that is, an X-ray mask having an X-ray absorber film with average film thickness of at least 5 .mu.m has long been in demand. Meanwhile, the average film thickness of the X-ray absorber film formed by the conventional method of manufacturing the X-ray mask is about 0.7 .mu.m and it is difficult to manufacture an X-ray mask having an X-ray absorber film with an average thickness of 3 .mu.m or thicker by the conventional method.